In commonly-assigned U.S. Pat. applications Ser. Nos. 07/451,655 now Pat. No. 5,164,742 and 07/451,656 now Pat. No. 5,164,742 both filed Dec. 18, 1989, a thermal printer is disclosed which may be adapted for use as a direct digital color proofer with halftone capabilities. This printer forms an image on a thermal print medium in which a donor element transfers a dye to a receiver element in response to a sufficient amount of thermal energy. It includes a plurality of diode lasers which can be individually modulated to supply energy to selected areas of the medium in accordance with an information signal. The printer includes a printhead which includes one end of a fiber optic array having a plurality of optical fibers coupled to the diode lasers. A thermal print medium is supported on a rotatable drum, and the printhead with the fiber optic array is movable relative to the drum. Dye is transferred by sublimation to the receiver element as the radiation, transferred from the diode lasers to the donor element by the optical fibers is converted to thermal energy in the donor element.
Commonly-assigned U.S. application 07/670,092 filed Mar. 15, 1991, describes a high quality version of the color proofer from the earlier applications, which is capable of consistently and accurately writing pixels at a rate of 1800 dots per inch and higher to generate halftone proofs having a resolution of 150 lines per inch and above. Each dot or mini-pixel is held to a density tolerance of better than 0.1 density unit from that prescribed in order to avoid visible differences between the original and the proof.
This application describes an autofocus device for continuously focusing the writing beam as the image is formed to assure that variations in the thickness of the donor and receiving elements as well as other perturbations in the system do not de-focus the writing beam and adversely affect the image density or the sharpness of the image. Although autofocus can be accomplished by reflecting the writing beam off the top surface of the donor element, far better results are obtained according to that application by reflecting a separate autofocus beam off an aluminized surface on the receiver which, in fact, is closer to the dye layers of the donor than is the front surface of the donor element. The autofocus beam is generated from a laser diode having an output between 900nm-1,050nm, for example, 960-970nm. The writing beams on the other hand are generated by laser diodes having outputs in the range of 800nm to 880nm, which wavelengths are absorbed by each of the donor members in order to effectively write the image. The autofocus beam, at 960-970nm passes through the donor members and is reflected off the aluminized surface of the receiver.
The autofocus beam and the writing beams are both presented to the apparatus in the printhead by optical fibers from their laser diodes. The ends of the optical fibers are imaged on the appropriate surfaces by a high numerical aperture lens. The light from the autofocus beam is reflected off the aluminized surface back through the lens to appropriate detectors to accomplish autofocus.